Phosphorus is IMO a tricky complex chemical to accurately determine for the average non-laboratory analyst. Accurately measuring phosphorus is not an easy chemical to accurately test compared to alkalinity, hardness, pH and nitrogen.
For testing phosphorus there are two basic types: “soluble” and is preferred over “dissolved” or “filterable” and the term “particulate” as chemically bound forms is preferred over “non-filterable.”
If samples are to be tested later, the samples should technically be preserved to reduce the phosphorus concentration from changing due to various biological and chemical reactions. Be aware that several substances can interfere with the phosphorus analysis in samples.
First, the type of phosphorus for testing is converted to dissolved orthophosphate. Second, the dissolved orthophosphate is then determined colorimetrically. If one visually evaluates the color change and does not use a colorimeter or spectrophotometer then the results are “ball park” range at best. Thus the accuracy of the results needed is determined by the type of test used.
Done correctly total phosphorus and total soluble phosphorus require a digestion step prior to the measurement of the orthophosphate form. The three digestion methods most commonly used during total phosphorus analysis include perchloric acid, nitric acid-sulfuric acid, and persulfate oxidation methods. Phosphorus occurs in association with organic matter, which must be effectively oxidized to release phosphorus as orthophosphate.
The amount of particulate (non-filterable) phosphorus or soluble (filterable) types depends on the type and size of filter (0.45um) used in the analytical procedure. While this filter pore size is small enough to exclude all bacteria, colloidal particles will still pass through the filter—meaning that very small chemical precipitant particles will pass through the filter paper and be measured as filterable or “soluble.”
https://www.waterrf.org/sites/default/files/file/2022-09/Measuring-Low-P-Concentrations.pdf
